1. Field of the Invention
The present invention relates to a control apparatus and method for driving an electric rotary machine such as a motor or a generator which is mounted on an electric vehicle.
2. Related Background Art
Not only industrial synchronous machines but also synchronous machines which have been widely used have a tendency that the terminal voltage of the synchronous machine, caused by a speed electromotive force, generally goes higher as the rotational speed of the synchronous machine increases. If this terminal voltage Vac goes over the maximum applied voltage Vtmax, the difference between Vac and Vtmax is applied to an inverter or power supply, which may damage a component. To prevent this, a field weakening control is performed which applies a predetermined negative field weakening current to the d axis to keep Vac equal to or less than Vtmax. This field weakening current is predetermined according to torques and rotational speeds of the machine so that Vac may not exceed Vtmax even when the battery voltage is low.
When the inverter-on voltage is ignored, the maximum A.C.(alternating current) voltage Vtmax which the power converter can apply is determined by a battery voltage VB as shown by Equation (1). Accordingly, if the battery voltage VB is high enough, the efficiency can be increased by reducing the magnitude of the field weakening current and the quantity of current flow. EQU Vtmax=(3/22)VB (1)
With regard to this point, Japanese Non-examined Patent Publication No.07-107772 discloses a method for increasing the efficiency without applying an excessive field weakening current. This method consists of always detecting a battery voltage, calculating a maximum impression voltage of the voltage converter by Equation (1), and determining the d-axis current command value Id* that makes the terminal voltage Vac of the synchronous motor equal to Vtmax by Equations (2) and (3) which represent a steady status. In Equation (2), R is a primary resistance, .omega. is an electric angular velocity of a motor, Ld is a d-axis inductance, Lq is a q-axis inductance, E0 is a number of flux interlinkages, Vd is a d-axis voltage, and Vq is a q-axis voltage.
##EQU1## Vac= (Vd.sup.2 +Vq.sup.2) (3)
An electric vehicle has two problems as the D.C. input voltage value temporarily varies due to consumption of battery power, driving of auxiliary units, and torque motoring/regeneration, and the dq-axis currents transit repeatedly and frequently.
First, a method disclosed in Japanese Non-examined Patent Publication No.07-107772 does not include the current transition status. At the time of a current transition, said Equation (2) has a term expressed by Equation (4) on its right side. Accordingly, the actual terminal voltage V1 of the synchronous motor becomes higher than Vac calculated by Equations (2) and (3) and it temporarily exceeds Vtmax. ##EQU2##
The second problem of the prior art is that the dq-axis currents are disturbed by a change of a D.C. input voltage value and a requested torque (torque command) cannot be accomplished exactly. For instance, when the D.C. input voltage drops down to 400V while a predetermined dq-axis current is applied at 500V of the D.C. input voltage, the A.C. voltage actually output from the power converter is multiplied by 400/500. Consequently, the dq-axis current value deviates from the predetermined value for a time period until the dq-axis compensating voltage command in the control equipment takes a value fit for the D.C. input voltage of 400V. Necessarily, the requested torque cannot be accomplished exactly.
In relation to the first problem, also when only a field weakening control is performed to keep the terminal voltage of the synchronous motor under a predetermined value, a battery power equivalent to the losses of the power converter and the synchronous motor is consumed, which reduces the distance per charging that the vehicle can run.